Method for stabilizing fluorine-containing polymer

ABSTRACT

A melt-processable fluorine-containing polymer is melt kneaded with a kneader which has a residence time of at least 10 minutes, a usable volume ratio (usable space in a container/space in a container) of larger than 0.3, and a power factor K of less than 8000, the power factor K being represented by the formula: K=Pv/μ/n 2  in which Pv is a power requirement per unit volume (W/m 3 ), μ is a melt viscosity (Pa.s), and n is a rotation speed (rps), to effectively remove terminal groups and bonds in the backbones, which are unstable during melt kneading, from the melt-processable fluorine-containing polymer, and obtain a colorless fluorine-containing polymer.

FIELD OF THE INVENTION

[0001] The present invention relates to a method for stabilizing afluorine-containing polymer. In particular, the present inventionrelates to a method for stabilizing a fluorine-containing polymer bytreating a fluorine-containing polymer which has unstable chain endsand/or unstable bonds in the backbones under specific conditions.

PRIOR ART

[0002] In the case of, for example, emulsion copolymers oftetrafluoroethylene and hexafluoropropylene, bubbles or voids may beformed from volatile materials in products produced by melt processing.The volatile materials are generated from the unstable chain ends andunstable backbones of the polymers, when heat or shear force is appliedto the polymers.

[0003] The kinds of unstable chain end groups vary with polymerizationmethods, and the kinds of polymerization initiators and chain transferagents. For example, carboxylic acid terminal groups are formed, when aconventional persulfate salt (e.g. ammonium persulfate, potassiumpersulfate, etc.) is used as a polymerization initiator in emulsionpolymerization. It is known that such carboxylic acid terminal groupsare the sources for volatile materials in the melt processing. Dependingon the conditions in the melt processing, groups such as olefinic groups(—CF═CF₂), acid fluoride groups (—COF) and the like are formed at thechain ends. These end groups may cause bubbles or voids in the finalproducts of the polymers.

[0004] Backbones which may generate volatile materials may be bondsbetween comonomers other than tetrafluoroethylene (TFE), as U.S. Pat.No. 4,626,587 describes. In the case oftetrafluoroethylene-hexafluoropropylene copolymers (FEP), the unstablebonds in the backbones are bonds between hexafluoropropylene monomers(HFP). This is confirmed form the fact that, when a gas generated byheating and melting FEP around 400° C. is analyzed, a molar ratio of HFPto TFE in the generated gas is about two times larger than that in thepolymers.

[0005] U.S. Pat. No. 4,626,587 proposes the removal of unstable chainend groups and unstable bonds in the backbones, which may be the causeof bubbles or voids found in the final products of fluorine-containingpolymers, by the application of a shear force with a twin-screwextruder. However, the use of a twin-screw extruder can remove theunstable bonds in the backbones because of the large shear force of theextruder, but hardly stabilizes the unstable end groups because of thetoo short residence time In addition, it is very difficult to removecoloring which appears because of the severe melting conditions, and theresidues of polymerization initiators or contamination. Thus, additionalstabilization treatment such as fluorination with other equipment isnecessary after the treatment with the twin-screw extruder. Furthermore,molded articles should be treated at a temperature lower than themelting point of the polymer, when the unstable end groups arestabilized after melt molding, since the shapes of the molded articlesshould be maintained.

SUMMARY OF THE INVENTION

[0006] An object of the present invention is to provide a method foreffectively removing unstable end groups and unstable bonds in backbonesfrom fluorine-containing polymers and also coloring, in the meltkneading step.

[0007] The above object can be achieved by a method for improving thethermal stability of a fluorine-containing polymer comprising meltkneading a melt-processable fluorine-containing polymer with a kneaderwhich has a residence time of at least 10 minutes, a usable volume ratio(usable space in a container/space in a container) of larger than 0.3,and a power factor K of less than 8000, the power factor K beingrepresented by the formula:

K=Pv/μ/n ²

[0008] wherein Pv is a power requirement per unit volume (W/m³), μ is amelt viscosity (Pa.s), and n is a rotation speed (rps).

DETAILED DESCRIPTION OF THE INVENTION

[0009] A kneader used in the method of the present invention isdistinguished from the above twin-screw extruder in that the kneader hasa longer residence time than the extruder, that is, the residence timeis usually at least 10 minutes, preferably between 10 and 120 minutes,and that the structures (e.g. usable volume ratios, etc.) and the powerfactors are different between them.

[0010] The conventional twin-screw has a usable volume ratio (usablespace in a container/space in a container) of 0.3 or less, while akneader which is preferably used in the present invention, that is, aso-called “surface renewal type kneader” has a usable volume ratio oflarger than 0.3, preferable at least 0.5. Herein, a usable space in acontainer means the space volume of a container in which paddles, ashaft, and the like are equipped, while a space in a container means aspace volume of a container not having paddles, a shaft, or the like.

[0011] The twin-screw extruder has a power factor K, which is defined bythe above formula, in the range between 8,000 and 12,000, while thesurface renewal type kneader has a power factor of less than 8,000,often 7,000 or less. The surface renewal type kneader has self-cleaningproperties, and high piston flow properties in continuous operation.

[0012] Typical examples of the surface renewal type kneaders includeHVR, SCR and NEW-SCR (all manufactured by Mitsubishi Heavy Industries,Ltd.), BIBOLACK (manufactured by Sumitomo Heavy Machinery andIndustries, Ltd.), HITACHI EYEGLASS-PADDLE POLYIMERIZER and HITACHIGATE-PADDLE POLYMERIZER (manufactured by Hitachi Ltd.), AP-MACHINE andNEW AP-MACHINE (manufactured by LIST), and the like.

[0013] Examples of the fluorine-containing polymers which are stabilizedby the method of the present invention include melt-processablecopolymers comprising at least two monomers selected from the groupconsisting of tetrafluoroethylene, hexafluoropropylene, perfluoroalkylvinyl ethers, ethylene, vinylidene fluoride and chlorotrifluoroethylene;vinylidene fluoride homopolymer; chlorotrifluoroethylene homopolymer;and the like.

[0014] The perfluoroalkyl vinyl ethers include a vinyl ether of theformula:

CF₂═CFO (CF₂)_(m)F

[0015] wherein m is an integer of 1 to 6, and a vinyl ether of theformula:

CF₂═CF(O—CF₂CF(CF₃))_(n)OC₃F₇

[0016] wherein n is an integer of 1 to 4.

[0017] In particular, when a fluorine-containing polymer, which istreated by the method of the present invention, is atetrafluoroethylene-hexafluoropropylene copolymer (FEP), it preferablycomprises 72 to 96 wt. % of tetrafluoroethylene and 4 to 28 wt. % ofhexafluoropropylene. When a fluorine-containing polymer is atetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), itpreferably comprises 92 to 99 wt. % of tetrafluoroethylene and 1 to 8wt. % of a perfluoroalkyl vinyl ether. When a fluorine-containingpolymer is a tetrafluoroethylene-ethylene copolymer (ETFE), itpreferably comprises 74.5 to 89.3 wt. % of tetrafluoroethylene and 10.7to 25.5 wt. % of ethylene.

[0018] These polymers may comprise other monomers in an amount such thatthe inherent properties of each copolymer do not deteriorate. Examplesof the other monomers include hexafluoropropylene (when thefluorine-containing polymer does not comprise hexafluoropropylene),perfluoroalkyl vinyl ethers (when the fluorine-containing polymer doesnot comprise a perfluoroalkyl vinyl ether), ethylene (when thefluorine-containing polymer does not comprise ethylene), vinylidenefluoride (when the fluorine-containing polymer does not comprisevinylidene fluoride), and chlorotrifluoroethylene (when thefluorine-containing polymer does not comprise chlorotrifluoroethylene).

[0019] The melt processable fluorine-containing polymers are preferablyprepared by emulsion or suspension polymerization. When the polymers areFEP, PFA, and copolymers of tetrafluoroethylene, hexafluoropropylene anda perfluoroalkyl vinyl ether, they have a melt viscosity in the rangebetween 0.1 and 100 kPa.s at 372° C.

[0020] The method of the present invention is carried out preferably ata temperature in the range between 200 and 450° C.

[0021] The method of the present invention requires a residence time ofat least 10 minutes to achieve the desired effects. When the residencetime is less than 10 minutes, it is difficult to obtain afluorine-containing polymer having sufficient heat stability and nocoloring.

[0022] The kneader used in the method of the present invention may be abatch apparatus or a continuous apparatus, and preferably has goodself-cleaning properties, and good piston flow properties in continuousoperation. When these properties of the kneader are insufficient, it maytake a long time for obtaining all the charged raw materials in thedesired states.

[0023] The polymer is preferably discharged from the kneader in thecontinuous operation with a single-screw extruder which has a vent forremoving gasses which are dissolved in the molten polymer.

[0024] For effectively removing the unstable end groups and unstablebonds in the backbones from the fluorine-containing polymers andimproving the heat stability of the polymer, one or more of thefollowing additional procedures may be combined with the above describedfundamental conditions of the method of the present invention:

[0025] a) Supplying pure fluorine gas, or fluorine gas which is dilutedto a suitable concentration, in a sufficient amount for removing all theunstable end groups into a kneader;

[0026] b) Supplying water or steam in a sufficient amount for removingall the unstable end groups into a kneader;

[0027] c) Adding salts or bases comprising alkali metals or alkalineearth metals, ammonia, amines or their salts, or alcohols to thefluorine-containing polymers, and then charging the polymer into akneader;

[0028] d) Adding salts or bases comprising alkali metals or alkalineearth metals, ammonia, amines or their salts, or alcohols to thefluorine-containing polymers, prior to or during any step of the methodof the present invention;

[0029] e) Allowing the fluorine-containing polymers in contact withinert gas for a sufficient time prior to the charging of the polymer ina kneader for removing substantially all the absorbed or adsorbed oxygenin the polymer, and then supplying the polymer in a kneader.

[0030] The treatment of the present invention can remove almost all theunstable end groups and unstable bonds in the backbones, and convert theunstable end groups to stable perfluoromethyl end groups (—CF₃),difluorohydride end groups (—CF₂H), acid amide end groups (—CONH₂) andmethyl ester end groups (—COOCH₃).

[0031] The amount of unstable end groups and stable end groups can bequantitatively measured by infrared spectrometry. Such a measuringmethod is disclosed in US Patent Nos. 3,085,083 and 4,675,380, thedisclosures of which are hereby incorporated by reference, andJP-A-4-20507. The number of end groups can be measured as the number per10⁶ carbon atoms by this measuring method.

[0032] The amount of materials which volatilize during the meltprocessing of the polymers can be assayed by the measurement of avolatile index, that is, a VI value, which is known. The measuringmethod of a VI value will be explained below.

[0033] Ten grams of a polymer sample is charged in a heat resistantcontainer, and placed in a glass vessel which is connected with a vacuumline.

[0034] The vessel is evacuated to a reduced pressure of 2 mmHg or less,and inserted in a high temperature block kept at 380° C. to achievethermal equilibrium.

[0035] The pressure change is recorded every ten minutes over 60minutes, and a VI value is calculated in accordance with the followingformula:

VI=(P ₄₀ −P ₀)×V/10/W

[0036] wherein P₀ and P₄₀ are a pressure prior to the insertion in thehigh temperature block and a pressure after 40 minutes from theinsertion in the high temperature block, respectively, V is the volume(ml) of the vessel, and W is the weight (g) of the sample.

[0037] The volatile index is preferably less than 25. When the volatileindex is larger than 25, bubbles or voids, which cause troubles in themelt processing, may form.

[0038] The degree of coloring depends on the severity of meltingconditions, residues of polymerization initiators, and presence ofcontamination. The main cause for the coloration is supposed to becarbon atoms which appear in the polymers at a temperature of 200° C. orhigher. This supposition may be reasonable, since the degree ofcoloration has substantially perfect correlation with the number ofunpaired electrons on the carbon atoms of the sample polymers, when thenumber of unpaired electrons on the carbon atoms of the sample polymershaving different degrees of coloring is measured by ESR.

[0039] The polymers have substantially no coloring, when the number ofunpaired electrons on the carbon atoms is between 0 and 1×10¹⁴ spins/g,preferably 5×10¹³ spins/g or less, in terms of a spin density, which ismeasured by ESR at 77K. The method of the present invention can easilyachieve such a spin density level.

[0040] The method of the present invention can effectively remove theunstable end groups and unstable bonds in the backbone during meltkneading, and provide colorless fluorine-containing polymers whileavoiding the complicated conventional method which comprises removingthe unstable bonds in the backbones with a twin-screw extruder, and thenremoving unstable end groups with other equipment, as disclosed in U.S.Pat. No. 4,626,587.

[0041] Now, the present invention will be illustrated by the followingexamples.

EXAMPLE 1

[0042] A FEP powder, which had been prepared by emulsion polymerizationusing ammonium persulfate (APS), had a melt viscosity of 2.8 kPa.s, andcontained 12 mole % of HFP, was treated as follows, and then the kindsand numbers of end groups of the obtained FEP, and a volatile index weremeasured.

[0043] The above FEP powder (1 kg) was charged in a surface renewal typekneader having an internal volume of 1 liter, a usable volume ratio of0.82 and a power factor K of 225 (“BIBOLACK” manufactured by SumitomoHeavy Machinery and Industries, Ltd.), and kneaded for 40 minutes at380° C. and 50 rpm, while passing pure water at a rate of 2.0 g/min. andan air at a rate of 0.3 NL/min.

[0044] The obtained polymer was a milk-white one with transparency.

[0045] The kinds and amounts of the end groups and the volatile indexes(VI) of the FEP powder before and after the treatment are shown inTable 1. Almost all the unstable end groups were removed, and thevolatile index was lower after the treatment.

[0046] The amount of unpaired electrons on the carbon atoms of thepolymer after the above treatment is shown in Table 1 in terms of a spindensity measured by ESR at 77K. The spin density was very low.

[0047] Substantially all the unstable end groups were removed, and alsothe volatile index (VI) was low.

EXAMPLE 2

[0048] FEP powder was treated in the same manner as in Example 1 exceptthat the internal air of the kneader was thoroughly replaced withnitrogen gas after the charging of the FEP powder, no water was addedduring the treatment, and a fluorine gas which was diluted with nitrogento a concentration of 7.6 mole % was passed at a rate of 0.3 NL/min. inplace of air, and the treating period of time was 60 minutes.

[0049] After the treatment, the fluorine gas in the internal space ofthe kneader was thoroughly replaced with nitrogen gas, and then thecontent was discharged from the kneader. The FEP powder after treatmentwas milk-white.

[0050] The kinds and amounts of the end groups and the volatile indexes(VI) of the FEP powder before and after the treatment are shown inTable 1. Almost all the unstable end groups were removed, and thevolatile index was lower after the treatment.

[0051] The amount of unpaired electrons on the carbon atoms of thepolymer after the above treatment is shown in Table 1 in terms of a spindensity measured by ESR at 77K. The spin density was very low. TABLE 1Example 1 Example 2 Before After Before After treatment treatmenttreatment treatment End groups -COF 0 0 (groups/ -COOH (m) 131 2 131 310⁶ C atoms -COOH (d) 677 5 677 4 VI >100 4.2 >100 4.5 Spin density(spins/g) — 2.7 × 10¹³ — 2.2 × 10¹³

Comparative Example

[0052] The same FEP powder as one used in Examples 1 and 2 waspelletized with a single-screw extruder having a screw diameter of 50 mmand a L/D ratio of 30 at a cylinder temperature of 380° C. The pelletswere fluorinated with fluorine gas, which had been diluted with nitrogengas to a concentration of 7.6 mole %, in an autoclave at 185° C. Thevolatile index was measured with varying the fluorination time. Theresults are shown in Table 2.

[0053] It is found that the fluorination time of about 8 hours isnecessary for achieving a volatile index of 25 or less.

[0054] The amount of unpaired electrons on the carbon atoms of thepolymer just after the extrusion was as high as 4.5×10¹⁵ spins/g interms of a spin density measured by ESR at 77K. TABLE 2 Fluorinationtime (hrs) VI 0 48 2 35 3 29 6 26 8 23

What is claimed is:
 1. A method for improving the thermal stability of afluorine-containing polymer comprising melt kneading a melt-processablefluorine-containing polymer with a kneader which has a residence time ofat least 10 minutes, a usable volume ratio based on usable space in acontainer/space in a container, said usable volume ratio being largerthan 0.3, and a power factor K of less than 8000, the power factor Kbeing represented by the formula: K=Pv/μ/n ² wherein Pv is a powerrequirement per unit volume (W/m³), μ is a melt viscosity (Pa.s), and nis a rotation speed (rps) wherein said fluorine-containing polymer is acopolymer comprising at least two monomers selected from the groupconsisting of tetrafluoroethylene, hexafluoropropylene, perfluoroalkylvinyl ethers, ethylene, vinylidene fluoride and chlorotrifluoroethylene;vinylidene fluoride homopolymer; or chlorotrifluoroethylene homopolymerwherein a pure fluorine gas or a diluted fluorine gas is supplied intothe kneader to remove any unstable end groups.
 2. A method according toclaim 1, wherein said kneader is a twin screw type kneader and has aself-cleaning mechanism and piston flow properties.
 3. A methodaccording to claim 1, wherein said melt-processable fluorine-containingpolymer is one prepared by emulsion or suspension polymerization, andthe amount of unpaired electrons on the carbon atoms of said polymer isreduced to 1×10¹⁴ spins/g in terms of a spin density measured by ESR at77K.
 4. A method according to claim 1, wherein said fluorine-containingpolymer is a copolymer of tetrafluoroethylene and hexafluoropropylene, acopolymer of tetrafluoroethylene and a perfluoroalkyl vinyl ether, or acopolymer of tetrafluoroethylene, hexafluoropropylene and aperfluoroalkyl vinyl ether, and has a melt viscosity in the rangebetween 0.1 and 100 kPa.s at 372° C.
 5. A method according to claim 1,which further comprises supplying fluorine gas in a sufficient amountfor removing all the unstable end groups into said kneader and allowingsaid fluorine-containing polymer in contact with inert gas from removingsubstantially all the absorbed or adsorbed oxygen in saidfluorine-containing polymer, prior to the supply of the fluorine gas.